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Your tank size is about adequate for a betta or a school of very small fish. In other words you are massively overstocked. Additionall, I see a powerhead but no filter. I also see no heater. I'm very sorry to say this but your tank is very likely to end in something you don't want, sickly fish dying.
The first signs of unhappy fish are already visible, the tetras are extremely pale!

My liter to gallons converter has 30L = 7.9 US Gal. For the fish you have it's a very small tank. Also, I see to different species being goldfish cold water and tiger barbs warm water. The white fish looks like Koi and if so they get huge. The tiger barbs are schooling fish and can be aggressive. If you had at least 10 (which is not recommended for your tank size) they may keep the aggression to themselves. Since you are now aware of the nitrogen cycle make sure you keep up with the water changes until the tank cycles.

My recommendation (if possible) would be return the fish and start a fishless cycle. While the cycle process is going go to http://aqadvisor.com/ and and research compatible species for your size tank. Hope this helps. The experts will definately give their 2 cents on the question.

You could well be right on the gallons. I've never been too good at doing imperial conversions in my head.

I don't know about goldfish but yes, this could be a koi I suppose:

This really looks like a blackskirt tetra

Despite the best intentions of doing a 20% daily returning the fish and starting over is probably the best bet. The filter does worry me though, perhaps it's the pictures but I don't see a sponge or anything.

That's all I think! Advice or recommendations are much appreciated :)
BTW, I'm extremely sorry if I posted this in the wrong section! >_<

Agree with all of the above. With all of those fish in that size tank, the inevitable will happen - the ammonia level in the tank will become very high (because there are too many fish, despite your water changes) - I suspect you don't own a test kit (to test your water parameters) which is necessary while your tank is cycling, especially if there are fish in it.

In addition, fish become stressed (and then ill) both from ammonia (from their waste) as well as not having enough space to swim and being crowded. Is is possible to return these fish and just cycle the tank without them? It appears you are truly interested in their well being, therefore, it would make sense to: 1) return them and cycle your current tank first 2) make a decision about what you want in it that's APPROPRIATE (checking with this forum 1st) or 3) get a larger tank (the largest one you can afford is what is recommended) that can accommodate the fish you want to keep - it is just not a good idea to get a tank, plop any old fish you want into it and expect it to be successful - fish grow and need a healthy environment.

If possible, all of them. It's best to go with the fishless cycle. You may want to look into getting a heater if you don't have one already. A larger filter rated for maybe a 16 gallon tank as it is recommended to have filtration rated at double the size of your tank.

Okay! I will do so. I'll return them and try fishless cycle. One more question, can a Betta fish survive alone? If not, I'll consider another type of fish, since I don't think 2 Betta fishes can survive in my tank.